1. Technical Field
The invention relates generally to the interconnection of integrated circuit structures, and more particularly to the formation of conductors within high aspect ratio apertures.
2. Background Art
As integrated circuit structures have become more compact, the need for low resistance metal connections between these structures has increased. Chemically vapor deposited (CVD) tungsten, evaporated aluminum doped with copper or silicon, and titanium or cobalt silicide have been used recently in tne industry to provide tnese interconnections.
It has been found that, in order to preserve the low resistivity of these metals, it is necessary to include some sort of barrier layer between the metal and the conductive structure to be contacted. These barrier layers prevent spiking between the metal and underlaying structures, while also preventing diffusion of nonconductive species that may penetrate into underlaying structures during the deposition of the metal. For example, during the deposition of CVD tungsten, fluorine byproducts of the chemical reduction of tungsten hexafluoride may penetrate lower layers and degrade resistivity characteristics. Moreover, when the underlaying structures include nonreactive insulators such as silicon oxide, barrier layers provide enhanced adhesion between the metal and the insulator. Finally, some barrier layers such as titanium help reduce Interfacial resistance by reacting with, and removing, native oxides and etch residuals from exposed interfaces.
One of the layers that has been used recently in the art is titanium nitride (TiN). Typically, the TiN barrier layer is formed by reactive sputtering from a titanium source in a nitrogen-containing ambient or directly from a titanium nitride source. Examples include U.S. Pat. No. 4,783,248, "Method For The Production Of A Titanium/Titanium Nitride Double Layer," to Kohlhase et al. and assigned to Siemens; U.S. Pat. No. 4,822,753, "Method For Making A W/TiN Contact," to Pintchovski et al. and assigned to Motorola; and U.S. Pat. No. 4,920,073, "Selective Silicidation Process Using A Titanium Nitride Protective Layer," to Wei et al. and assigned to Texas Instruments.
Conventional sputtering provides satisfactory results when used on a planar surface. Moreover, it also is useful when used to coat the sidewalks and bottom of an aperture (or via) formed through a passivation or other insulating layer to an underlaying structure, where the ratio of the height of the aperture to its width (hereinafter the "aspect ratio" of the via) is less than 1:1. However, as the aspect ratio of the via increases, conventional sputtering does not provide acceptable results. Specifically, far less material is deposited at the Dottom portions of the via or hole than at the top, since the walls "shadow" the lower exposed surface. As a result, deposited material at the upper surfaces increasingly accentuates the shadowing effect, thereDy prematurely closing the upper section of the structure to be filled and preventing effective fill of the lower section.
This problem is illustrated in FIG. 5 of U.S. Pat. No. 4,897,709, "Titanium Nitride Film In Contact Hole With Large Aspect Ratio," to Yokoyama et al. and assigned to Hitachi. The sputtered titanium nitride is 135 nm thick on the upper surface of the passivation layer and only 40 nm thick at the bottom of the contact. Also, note that portions of the titanium nitride on the upper surface of the passivation layer extend into the contact hole to form rounded deposits on the upper sidewalls of the contact hole. This deposit will tend to close of the contact hole before it is completely filled by either sputtered or CVD deposited conductor films.
The solution disclosed by Yokoyama et al. is to deposit the titanium nitride in a plasma CVD reaction. By its very nature, CVD tends to be more conformal, such that the amount of material at the bottom of the contact hole will be similar to the amount on the upper norizontal surfaces. This will also prevent the formation of the aforementioned rounded deposits. Unfortunately, the deposition is carried out using chlorinated titanium species such as titanium tetrachloride. During the titanium reduction reaction, chlorinated reaction products may be incorporated into the titanium nitride, substantially reducing the low contact resistance benefits afforded by titanium nitride. Further, the omission of a pure Ti layer from the structure means that native oxides or other nonconductive residuals will not be removed from the bottom of the contact hole.